package/network-manager: drop unrecognized --without-docs

--without-docs was not recognized since at least version 1.8.4, see:
http://autobuild.buildroot.org/results/e1464adf0069952ef0561a0a9b501d26bd409c4a



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>